Thermal sensor circuit

ABSTRACT

An electronic device includes a module that delivers a positive temperature coefficient output voltage at an output terminal. A thermistor includes a first MOS transistor operating in weak inversion mode and having a negative temperature coefficient drain-source resistance and whose source is coupled to the output terminal. A current source coupled to the output terminal operates to impose the drain-source current of the first transistor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application forPatent No. 62/985,992, filed Mar. 6, 2020, the disclosure of which isincorporated by reference.

TECHNICAL FIELD

Embodiments relate to electronic devices and, in particular, to athermal sensor circuit implemented as an integrated electronic circuitsupported by a bulk semiconductor substrate, a partially depletedsemiconductor on insulator (PD-SOI) substrate, or a fully depletedsemiconductor on insulator (FD-SOI).

BACKGROUND

Reference is made to FIG. 1 showing a circuit diagram for an embodimentof a temperature sensing circuit module 3 as taught by United StatesApplication for patent Ser. No. 16/572,130 (incorporated by reference).The circuit module 3 includes: a thermistor 4 and a current source 5.The thermistor 4 is formed by a first MOS transistor T1 (comprising, forexample, an NMOS transistor) having a source S1 that is coupled to anoutput terminal BS of the circuit module 3 and a drain D1 and a gate G1that are electrically shorted to each other and coupled to a supplyvoltage VDD. The current source 5 is formed by a second MOS transistorT2 (comprising, for example, an NMOS transistor) having a gate G2 thatis coupled to the output terminal BS (and thus to the source S1) and adrain D2 and a source S2 that are each coupled to a ground voltage GND.

The transistors T1 and T2 are each supported by a semiconductor oninsulator (SOI) substrate (FIG. 1A), and thus the first MOS transistorT1 further includes a back gate B1 and the second MOS transistor T2further includes a back gate B2. As understood by those skilled in theart, an SOI substrate is formed, for example, by semiconductor (forexample, silicon) film layer that is insulated from a semiconductor (forexample, silicon) support layer by an insulating layer (typicallyreferred to in the art as a buried oxide (BOX) layer). The sources S1,S2 and drains D1, D2 are formed by doped regions in the semiconductorfilm layer and the back gates B1 and B2 are formed by doped portions ofthe semiconductor support layer located underneath the channel region ofthe transistor.

In particular, the semiconductor on insulator (SOI) used to support thefirst and second transistors T1, T2 of the module 3 includes asemiconductor film layer that is of the fully depleted type. Fullydepleted semiconductor on insulator (FD-SOI) is a planar processtechnology that relies on two innovations. The first innovation concernsthe use of a very thin BOX layer (for example, with a thickness in therange of 5-50 nm). The second innovation concerns the use of a very thinsemiconductor film layer (for example, with a thickness in the range of5-20 nm). With this configuration, the depletion region can cover thewhole thickness of the semiconductor film layer because thesemiconductor material for the semiconductor film layer is undoped(intrinsic) or very lightly doped and there are no (or very few) mobilecharges in the channel region of the transistor. The back gates B1, B2of the first and second transistors T1, T2, respectively, are coupled tothe ground supply voltage GND and are not used to control the operationof the first and second transistors T1 and T2.

The voltage at the supply voltage VDD may, for example, be about 1.0V(and in particular may be 0.9V for the FD-SOI implementation), so thatthe first transistor T1 is in weak inversion mode, that is to say havinga gate-source voltage lower than its threshold voltage.

In this case, the output voltage Vout at the output terminal BS, inother words the gate voltage Vg2 of the second transistor T2, isvariable between 0.6 V and 0.95 V, thereby allowing operation of thesecond transistor T2 in strong inversion mode, that is to say having agate-source voltage higher than its threshold voltage.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

As the second transistor T2 is operating in strong inversion mode, thereis a gate leakage current Ig2 of the second transistor T2 that imposesthe drain-source current Ids1 of the first transistor T1. The orders ofmagnitude of the gate leakage current Ig2 and of the drain-sourcecurrent Ids1 of the first transistor T1 are substantially equal. By wayof indication, these leakage currents are of the order of about onehundred picoamperes.

To this end, the thickness of the gate oxide of the second transistor T2is relatively thin and is preferably less than 2 nm.

FIG. 2 illustrates operation of the module 3 as a function of thetemperature, within a range between −150° C. and 200° C., and withVDD=0.9V to 1.0V. The curve CB1 shows results of a SPICE (“SimulationProgram with Integrated Circuit Emphasis”) simulation of the module 3 asa function of the temperature, within a range between −150° C. and 200°C. The curve CB2 shows results for implementation of the module 3 at thetechnology node 28 FD-SOI (characterized, for example, by a 28 nmprocess with a high-k metal gate and ultra-thin box and body (UTBB)substrate with a thin (for example, 1.5 nm equivalent oxide thickness(EOT)) oxide front gate and epitaxial raised source/drain structures).

As is seen from the curves CB1 and CB2 of FIG. 2, the output voltageVout varies proportionally to the temperature. For example, the outputvoltage Vout for curve CB1 varies substantially linearly between 0.6Vand 0.95V (for VDD=1.0V). Similarly, the output voltage Vout for curveCB2 varies substantially linearly between 0.5V and 0.85V (for VDD=0.9V).Thus, the module 3 forms an accurate and useful temperature sensingcircuit, with the output voltage at node BS having a value proportionalto temperature (i.e., the output voltage has a positive temperaturecoefficient).

It has been noted, however, that similar substantially linear curves foroutput voltage versus temperature are not necessarily obtained when thecircuit module 3 is extended to other technology nodes (includingdifferent substrate types).

For example, consider an extension to fabrication of the circuit modulefor a bulk semiconductor substrate technology node. FIG. 3 shows acircuit diagram for an embodiment of a temperature sensing circuitmodule 3 a. The first MOS transistor T1 forming the thermistor 4 has asource S1 that is coupled to the output terminal BS of the circuitmodule 3 and a drain D1 and a gate G1 that are electrically shorted toeach other and coupled to the supply voltage VDD. The second MOStransistor T2 forming the current source 5 has a gate G2 that is coupledto the output terminal BS (and thus to the source S1) and a drain D2 anda source S2 that are each coupled to the ground voltage GND. Thetransistors T1 and T2 are each supported by a bulk semiconductor (forexample, silicon) substrate as shown in FIG. 3A. In this implementation,there are no back gates for the transistors T1 and T2, and the body(bulk) of the transistor T1 is tied to the source S1 of transistor T1and the body (bulk) of the transistor T2 is either electrically tied tothe ground voltage GND or tied to the source S2 of transistor T2.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 4 illustrates operation of the module 3 a as a function of thetemperature, within a range between −150° C. and 200° C. The curve CB3shows results for implementation of the module 3 a at the technologynode 40 bulk (characterized, for example, by a 40 nm process with apolysilicon gate having a thin (for example, 1.7 nm) dimension withVDD=1.1V). The curve CB4 shows results for implementation of the module3 a at the technology node 65 bulk (characterized, for example, by a 40nm process with a polysilicon gate having a thin (for example, 1.8 nm)dimension with VDD=1.2V). The curve CB2 which shows results forimplementation of the module 3 at the technology node 28 FD-SOI, withVDD=0.9V, is provided for comparison purposes. It will be noted thatthere is significant linearity degradation for the module 3 a especiallyin the temperature range from −40 to +125° C.

As a further example, consider an extension to fabrication of thecircuit module for a partially depleted semiconductor on insulator(PD-SOI) substrate technology node. FIG. 5A shows a circuit diagram foran embodiment of a temperature sensing circuit module 3 b 1. The firstMOS transistor T1 forming the thermistor 4 has a source S1 that iscoupled to the output terminal BS of the circuit module 3 and a drain D1and a gate G1 that are electrically shorted to each other and coupled tothe supply voltage VDD. The second MOS transistor T2 forming the currentsource 5 has a gate G2 that is coupled to the output terminal BS (andthus to the source S1) and a drain D2 and a source S2 that are eachcoupled to the ground voltage GND.

The transistors T1 and T2 are each supported by a partially depletedsemiconductor on insulator (FD-SOI) substrate. PD-SOI is a planarprocess technology that is generally structurally similar to FD-SOI, butdiffers in that both the BOX layer and semiconductor film layer arethicker. With this configuration, the depletion region will not coverthe whole thickness of the semiconductor film layer. The semiconductormaterial for the semiconductor film layer in PD-SOI is also typicallylightly doped. Preferably, the semiconductor film layer has a thicknessthat is not too great so as to avoid memory effects, for example athickness close to that of the semiconductor film of the FD-SOIsubstrate.

In this implementation, the back gates B1 and B2 for the transistors T1and T2, respectively, are electrically connected to the ground voltageGND. The body (i.e., the channel region of the semiconductor film layer)of the transistor T1 and the body (i.e., the channel region of thesemiconductor film layer) of the transistor T2 are both left floating(i.e., the body region of the substrate is not electrically connected toa bias voltage or to a transistor source). This implementation isessentially the same as the implementation for module 3 of FIG. 1.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 6A illustrates operation of the module 3 b 1 as a function of thetemperature, within a range between −150° C. and 200° C., with VDD=1.2V.The curve CB5-1 shows results for implementation of the module 3 b 1 atthe technology node H9 SOI (characterized, for example, by a 130 nmprocess with a polysilicon gate having a thin (for example, 2 nm)dimension). The curve CB6-1 shows results for implementation of themodule 3 b 1 at the technology node 65 SOI (characterized, for example,by a 65 nm process with a polysilicon gate having a thin (for example,1.8 nm) dimension). The curve CB2 which shows results for implementationof the module 3 at the technology node 28 FD-SOI, with VDD=0.9V, isprovided for comparison. It will be noted that there is significantlinearity degradation for the module 3 b 1 especially in the temperaturerange from −40 to +125° C.

FIG. 5B shows a circuit diagram for an embodiment of a temperaturesensing circuit module 3 b 2. The first MOS transistor T1 forming thethermistor 4 has a source S1 that is coupled to the output terminal BSof the circuit module 3 and a drain D1 and a gate G1 that areelectrically shorted to each other and coupled to the supply voltageVDD. The second MOS transistor T2 forming the current source 5 has agate G2 that is coupled to the output terminal BS (and thus to thesource S1) and a drain D2 and a source S2 that are each coupled to theground voltage GND. The transistors T1 and T2 are each supported by aPD-SOI substrate. In this implementation, the back gates B1 and B2 forthe transistors T1 and T2, respectively, are electrically connected tothe ground voltage GND. The body (i.e., the channel region of thesemiconductor film layer) of the transistor T1 is tied to the source S1of the transistor T1. The body (i.e., the channel region of thesemiconductor film layer) of the transistor T2 is electrically connectedto the ground voltage GND.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 6B illustrates operation of the module 3 b 2 as a function of thetemperature, within a range between −150° C. and 200° C., with VDD=1.2V.The curve CB5-2 shows results for implementation of the module 3 b 2 atthe technology node H9 SOI. The curve CB6-2 shows results forimplementation of the module 3 b 2 at the technology node 65 SOI. Thecurve CB2 which shows results for implementation of the module 3 at thetechnology node 28 FD-SOI is provided for comparison. It will be notedthat there is significant linearity degradation for the module 3 b 2especially in the temperature range from −40 to +125° C.

There is accordingly a need in the art to address the issue of variationin output voltage versus temperature performance of the circuit module 3when extended across technology nodes.

SUMMARY

An embodiment of a temperature sensor includes: a thermistor formed byat least a first MOS transistor configured to operate in weak inversionmode and have a negative temperature coefficient drain-sourceresistance. The source of the first MOS transistor is coupled to anoutput terminal. A current source formed by a second MOS transistor hasa control terminal coupled to the source of the first MOS transistor.The second MOS transistor is configured to operate in strong inversionmode and have a gate leakage current which imposes the drain-sourcecurrent of the first MOS transistor. The gate leakage current is of thesame order of magnitude as the drain-source current of the first MOStransistor.

When the first MOS transistor operates in weak inversion mode, alsoknown to those skilled in the art as operating in sub-threshold mode,the gate voltage of the MOS first transistor is lower than the thresholdvoltage of the first transistor, and is biased with a fixed drain-sourcevoltage. The drain-source current output by the first MOS transistor hasa positive temperature coefficient (i.e., the drain-source resistance ofthe first MOS transistor has a negative temperature coefficient, andthus decreases as the temperature increases). With the drain voltage ofthe first transistor fixed and with the drain-source current imposed bythe second MOS transistor, the drain-source voltage of the first MOStransistor, which is equal to the product of the drain-source currentand the drain-source resistance, decreases as the temperature increases.As a result, the source voltage of the first transistor, in other wordsthe voltage at the output terminal, increases as the temperatureincreases, and therefore has a positive temperature coefficient.

Advantageously, such a temperature-sensitive module comprising two MOStransistors (and perhaps a third MOS transistor in an alternativeembodiment) has a compact structure. Furthermore, because the thermistoris operating in weak inversion mode, the drain-source current of thefirst MOS transistor (and third MOS transistor coupled in seriestherewith) is very low, and the power consumption of thetemperature-sensitive module is therefore advantageously low.

The first and second MOS transistors may, for example, be NMOStransistors. The third MOS transistor may also be an NMOS transistor.

By way of non-limiting example, the temperature sensor may be producedin an integrated manner.

The temperature sensor may, for example, be produced on a bulk substrateor a silicon-on-insulator substrate. When said substrate is asilicon-on-insulator substrate, the silicon-on-insulator substrate may,for example, be a fully or partially depleted silicon-on-insulatorsubstrate.

The temperature sensor may, for example, be a circuit component offunctional circuitry for an electronic device, such as a cellular mobiletelephone, tablet or laptop computer.

In an embodiment, a temperature sensing circuit comprises: a thermistorcomprising a first MOS transistor having a first drain, a first source,a first gate and a first body, wherein the first MOS transistor isconfigured to operate in weak inversion mode with a negative temperaturecoefficient drain-source resistance; an output terminal coupled to thefirst source and configured to generate a positive temperaturecoefficient output voltage; and a current source comprising a second MOStransistor having a second drain, a second source, a second gate and asecond body, wherein the second gate is coupled to the first source andconfigured to impose a drain-source current of the first MOS transistor;and wherein the first body and second body are directly electricallyconnected to a ground node.

In an embodiment, a temperature sensing circuit comprises: a thermistorcomprising a first MOS transistor having a first drain, a first source,a first gate, a first body and a first back gate, wherein the first MOStransistor is configured to operate in weak inversion mode with anegative temperature coefficient drain-source resistance; an outputterminal coupled to the first source and configured to generate apositive temperature coefficient output voltage; and a current sourcecomprising a second MOS transistor having a second drain, a secondsource, a second gate, a second body and a second back gate, wherein thesecond gate is coupled to the first source and configured to impose adrain-source current of the first MOS transistor; and wherein the firstbody and second body are floating, and wherein the first back gate andthe second back gate are directly electrically connected to a groundnode.

In an embodiment, a temperature sensing circuit comprises: a thermistorcomprising a first MOS transistor having a first drain, a first source,a first gate and a first body, wherein the first MOS transistor isconfigured to operate in weak inversion mode with a negative temperaturecoefficient drain-source resistance; an output terminal coupled to thefirst source and configured to generate a positive temperaturecoefficient output voltage; a current source comprising a second MOStransistor having a second drain, a second source, a second gate and asecond body, wherein the second gate is coupled to the first source andconfigured to impose a drain-source current of the first MOS transistor;a bias voltage generator configured to generate a bias voltage; andwherein the first body is configured to receive the bias voltage, andwherein the second body is directly electrically connected to a groundnode.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become apparent uponexamining the detailed description of completely non-limitingembodiments and the appended drawings, in which:

FIG. 1 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 1A generally shows a semiconductor on insulator (SOI) substratesupporting a MOS transistor;

FIG. 2 illustrates operation of the temperature sensing circuit moduleof FIG. 1 as a function of voltage versus temperature;

FIG. 3 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 3A generally shows a bulk substrate supporting a MOS transistor;

FIG. 4 illustrates operation of the temperature sensing circuit moduleof FIG. 3 as a function of voltage versus temperature;

FIG. 5A is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 6A illustrates operation of the temperature sensing circuit moduleof FIG. 5A as a function of voltage versus temperature;

FIG. 5B is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 6B illustrates operation of the temperature sensing circuit moduleof FIG. 5B as a function of voltage versus temperature;

FIG. 7 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 8 illustrates operation of the temperature sensing circuit moduleof FIG. 7 as a function of voltage versus temperature;

FIG. 9 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIGS. 10A-10B illustrate operation of the temperature sensing circuitmodule of FIG. 9 as a function of voltage versus temperature;

FIG. 11 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 12 illustrates operation of the temperature sensing circuit moduleof FIG. 11 as a function of voltage versus temperature;

FIG. 13 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 14 illustrates operation of the temperature sensing circuit moduleof FIG. 13 as a function of voltage versus temperature;

FIG. 15 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIGS. 16A-16B illustrate operation of the temperature sensing circuitmodule of FIG. 15 as a function of voltage versus temperature;

FIG. 17 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 18 illustrates operation of the temperature sensing circuit moduleof FIG. 17 as a function of voltage versus temperature;

FIG. 19 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIG. 20 illustrates operation of the temperature sensing circuit moduleof FIG. 19 as a function of voltage versus temperature;

FIG. 21 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIGS. 22A-22B illustrate operation of the temperature sensing circuitmodule of FIG. 21 as a function of voltage versus temperature;

FIG. 23 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIGS. 24-25 illustrate operation of the temperature sensing circuitmodule of FIG. 23 as a function of voltage versus temperature and inresponse to a controlled body bias voltage;

FIG. 26 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIGS. 27-28 illustrate operation of the temperature sensing circuitmodule of FIG. 26 as a function of voltage versus temperature and inresponse to a controlled body bias voltage;

FIG. 29 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIGS. 30-31 illustrate operation of the temperature sensing circuitmodule of FIG. 29 as a function of voltage versus temperature and inresponse to a controlled body bias voltage;

FIG. 32 is a circuit diagram for an embodiment of a temperature sensingcircuit module;

FIGS. 33-34 illustrate operation of the temperature sensing circuitmodule of FIG. 32 as a function of voltage versus temperature and inresponse to a controlled body bias voltage; and

FIG. 35 illustrates an electronic appliance with a temperature sensingcircuit module.

DETAILED DESCRIPTION

In connection with fabrication of the circuit module for a bulksemiconductor substrate technology node, FIG. 7 shows a circuit diagramfor an embodiment of a temperature sensing circuit module 3 c. The firstMOS transistor T1 forming the thermistor 4 has a source S1 that iscoupled to the output terminal BS of the circuit module 3 and a drain D1and a gate G1 that are electrically shorted to each other and coupled tothe supply voltage VDD. The second MOS transistor T2 forming the currentsource 5 has a gate G2 that is coupled to the output terminal BS (andthus to the source S1) and a drain D2 and a source S2 that are eachcoupled to the ground voltage GND. The transistors T1 and T2 are eachsupported by a bulk semiconductor (for example, silicon) substrate (see,FIG. 3A). In this implementation, there are no back gates for thetransistors T1 and T2. The body (bulk) of the transistor T1 is directlyelectrically connected to the ground voltage GND and the body (bulk) ofthe transistor T2 is also directly electrically connected to the groundvoltage GND.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 8 illustrates operation of the module 3 c as a function of thetemperature, within a range between −150° C. and 200° C. The curve CB3′shows results for implementation of the module 3 c at the technologynode 40 bulk, with VDD=1.1V. The curve CB4′ shows results forimplementation of the module 3 c at the technology node 65 bulk, withVDD=1.2V. For comparison purposes, the curve CB2 for module 3 and thecurves CB3 and CB4 for module 3 a are also shown. It will be noted thatthere is some improvement in linearity, especially for the module 3 cfabricated at the technology node 40 bulk, but linearity degradationnonetheless persists over some temperature ranges. The module 3 c isaccordingly an improvement over the module 3 a and is useful in somedevice applications.

In connection with fabrication of the circuit module for a partiallydepleted semiconductor on insulator (PD-SOI) substrate technology node,FIG. 9 shows a circuit diagram for an embodiment of a temperaturesensing circuit module 3 d. The first MOS transistor T1 forming thethermistor 4 has a source S1 that is coupled to the output terminal BSof the circuit module 3 and a drain D1 and a gate G1 that areelectrically shorted to each other and coupled to the supply voltageVDD. The second MOS transistor T2 forming the current source 5 has agate G2 that is coupled to the output terminal BS (and thus to thesource S1) and a drain D2 and a source S2 that are each coupled to theground voltage GND. The transistors T1 and T2 are each supported by aPD-SOI substrate (see, FIG. 1A). In this implementation, the back gatesB1 and B2 for the transistors T1 and T2, respectively, are directlyelectrically connected to the ground voltage GND. Furthermore, the body(i.e., the channel region of the semiconductor film layer) of thetransistor T1 and the body (i.e., the channel region of thesemiconductor film layer) of the transistor T2 are also directlyelectrically connected to the ground voltage GND.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIGS. 10A-10B illustrate operation of the module 3 d as a function ofthe temperature, within a range between −150° C. and 200° C., withVDD=1.2V. The curve CB5′ shows results for implementation of the module3 d at the technology node H9 SOI. The curve CB6′ shows results forimplementation of the module 3 d at the technology node 65 SOI. Forcomparison purposes, the curve CB2 for the module 3 and the curves CB5-2and CB6-2 for the module 3 b 2 are also shown. It will be noted thatthere is some improvement in linearity, but linearity degradationnonetheless persists over some temperature ranges. The module 3 d isaccordingly an improvement over the module 3 b 2 and is useful in somedevice applications.

In connection with fabrication of the circuit module for a bulksemiconductor substrate technology node, FIG. 11 shows a circuit diagramfor an embodiment of a temperature sensing circuit module 3 e. The firstMOS transistor T1 forming the thermistor 4 has a source S1 that iscoupled to the output terminal BS of the circuit module 3 and a drain D1and a gate G1 that are electrically shorted to each other and coupled tothe supply voltage VDD. The second MOS transistor T2 forming the currentsource 5 has a gate G2 that is coupled to the output terminal BS (andthus to the source S1) and a drain D2 and a source S2 that are eachcoupled to the ground voltage GND. The transistors T1 and T2 are eachsupported by a bulk semiconductor (for example, silicon) substrate (see,FIG. 3A). In this implementation, there are no back gates for thetransistors T1 and T2. The body (bulk) of the transistor T1 is directlyelectrically connected to the ground voltage GND and the body (bulk) ofthe transistor T2 is also directly electrically connected to the groundvoltage GND. Additionally, the transistor T1 is fabricated to have athicker gate oxide. For example, the gate oxide of transistor T1 in themodule 3 e has a thickness of about 5 nm, while the gate oxide oftransistor T1 in the module 3 c has a thickness of 1.7-1.8 nm.Increasing the thickness of the gate oxide of transistor T1 has theeffect of improving the gate-body (GB) and drain-body (DB) leakage andsub-threshold voltage performance.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 12 illustrates operation of the module 3 e as a function of thetemperature, within a range between −150° C. and 200° C. The curve CB3″shows results for implementation of the module 3 e at the technologynode 40 bulk, with VDD=1.1V. The curve CB4″ shows results forimplementation of the module 3 e at the technology node 65 bulk, withVDD=1.2V. For comparison purposes, the curve CB2 for the module 3 andthe curves CB3′ and CB4′ for the module 3 c are also shown. It will benoted that there is some improvement in linearity, especially for themodule 3 e fabricated at the technology node 40 bulk, but linearitydegradation nonetheless persists over some temperature ranges. Themodule 3 e is accordingly an improvement over the module 3 c and isuseful in some device applications.

In connection with fabrication of the circuit module for a fullydepleted semiconductor on insulator (FD-SOI) substrate technology node,FIG. 13 shows a circuit diagram for an embodiment of a temperaturesensing circuit module 3 f. The first MOS transistor T1 forming thethermistor 4 has a source S1 that is coupled to the output terminal BSof the circuit module 3 and a drain D1 and a gate G1 that areelectrically shorted to each other and coupled to the supply voltageVDD. The second MOS transistor T2 forming the current source 5 has agate G2 that is coupled to the output terminal BS (and thus to thesource S1) and a drain D2 and a source S2 that are each coupled to theground voltage GND. The transistors T1 and T2 are each supported by aFD-SOI substrate (see, FIG. 1A). In this implementation, the back gatesB1 and B2 for the transistors T1 and T2, respectively, are directlyelectrically connected to the ground voltage GND. Furthermore, the body(i.e., the channel region of the semiconductor film layer) of thetransistor T1 and the body (i.e., the channel region of thesemiconductor film layer) of the transistor T2 are left floating. Inaddition, the transistor T1 is fabricated to have a thicker gate oxide.For example, the gate oxide of transistor T1 in the module 3 has athickness of about 1.5 nm, while the gate oxide of transistor T1 in themodule 3 f has a thickness of about 5 nm. Increasing the thickness ofthe gate oxide of transistor T1 has the effect of improving thegate-body (GB) and drain-body (DB) leakage and sub-threshold voltageperformance.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 14 illustrates operation of the module 3 f as a function of thetemperature, within a range between −150° C. and 200° C., and VDD=0.9V.The curve CB2′ shows results for implementation of the module 3 f . Forcomparison purposes, the curve CB2 for the operation of module 3 is alsoshown. It will be noted that module 3 f exhibits substantially the samelinearity with a slight improvement (i.e., increase) in slope. Themodule 3 f is accordingly an improvement over the module 3.

In connection with fabrication of the circuit module for a partiallydepleted semiconductor on insulator (PD-SOI) substrate technology node,FIG. 15 shows a circuit diagram for an embodiment of a temperaturesensing circuit module 3 g. The first MOS transistor T1 forming thethermistor 4 has a source S1 that is coupled to the output terminal BSof the circuit module 3 and a drain D1 and a gate G1 that areelectrically shorted to each other and coupled to the supply voltageVDD. The second MOS transistor T2 forming the current source 5 has agate G2 that is coupled to the output terminal BS (and thus to thesource S1) and a drain D2 and a source S2 that are each coupled to theground voltage GND. The transistors T1 and T2 are each supported by aPD-SOI substrate (see, FIG. 1A). In this implementation, the back gatesB1 and B2 for the transistors T1 and T2, respectively, are directlyelectrically connected to the ground voltage GND. Furthermore, the body(i.e., the channel region of the semiconductor film layer) of thetransistor T1 and the body (i.e., the channel region of thesemiconductor film layer) of the transistor T2 are also directlyelectrically connected to the ground voltage GND. In addition, thetransistor T1 is fabricated to have a thicker gate oxide. For example,the gate oxide of transistor T1 in the module 3 g has a thickness ofabout 5 nm, while the gate oxide of transistor T1 in the module 3 d hasa thickness of 1.8-2 nm. Increasing the thickness of the gate oxide oftransistor T1 has the effect of improving the gate-body (GB) anddrain-body (DB) leakage and sub-threshold voltage performance.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIGS. 16A-16B illustrate operation of the module 3 g as a function ofthe temperature, within a range between −150° C. and 200° C., withVDD=1.2V. The curve CB5″ shows results for implementation of the module3 g at the technology node H9 SOI. The curve CB6″ shows results forimplementation of the module 3 g at the technology node 65 SOI. Forcomparison purposes, the curve CB2 for the module 3 and the curves CB5′and CB6′ for the module 3 d are also shown. It will be noted that thereis some improvement in linearity, although not for module 3 gimplemented the technology node 65 SOI. The module 3 g is accordingly animprovement over the module 3 d and is useful in some deviceapplications.

In connection with fabrication of the circuit module for a bulksemiconductor substrate technology node, FIG. 17 shows a circuit diagramfor an embodiment of a temperature sensing circuit module 3 h. In thisembodiment, the thermistor 4 is implemented using the transistor T1coupled in series with a transistor T3 (comprising, for example, an NMOStransistor) having a source S3 that is coupled to the output terminal BSof the circuit module 3 and a drain D3 and a gate G3 that areelectrically shorted to each other and coupled to the source S1 oftransistor T1. The transistors T1, T2 and T3 are each supported by abulk semiconductor (for example, silicon) substrate (see, FIG. 3A). Inthis implementation, there are no back gates for the transistors T1, T2and T3. The body (bulk) of the transistor T1 is directly electricallyconnected to the ground voltage GND, the body (bulk) of the transistorT2 is also directly electrically connected to the ground voltage GND andthe body (bulk) of the transistor T3 is directly electrically connectedto the ground voltage GND. Additionally, the transistors T1 and T3 arefabricated to have a thicker gate oxide. For example, the gate oxide oftransistors T1 and T3 in the module 3 h have thicknesses of about 5 nm,while the gate oxide of transistor T1 in the module 3 e has a thicknessof 1.7-1.8 nm. Increasing the thickness of the gate oxide of transistorsT1 and T3 has the effect of improving the gate-body (GB) and drain-body(DB) leakage and sub-threshold voltage performance.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 18 illustrates operation of the module 3 h as a function of thetemperature, within a range between −150° C. and 200° C. The curve CB3′″shows results for implementation of the module 3 h at the technologynode 40 bulk, with VDD=1.1V. The curve CB4′″ shows results forimplementation of the module 3 h at the technology node 65 bulk, withVDD=1.2V. For comparison purposes, the curve CB2 for module 3 and thecurves CB3″ and CB4″ for module 3 e are also shown. It will be notedthat there is some improvement in slope, however, the curve CB4′″ showsan increase in the square coefficient of the second order polynomialtrend line. The module 3 h is accordingly an improvement over the module3 e and is useful in some device applications.

In connection with fabrication of the circuit module for a fullydepleted semiconductor on insulator (FD-SOI) substrate technology node,FIG. 19 shows a circuit diagram for an embodiment of a temperaturesensing circuit module 3 i. In this embodiment, the thermistor 4 isimplemented using the transistor T1 coupled in series with a transistorT3 (comprising, for example, an NMOS transistor) having a source S3 thatis coupled to the output terminal BS of the circuit module 3 and a drainD3 and a gate G3 that are electrically shorted to each other and coupledto the source S1 of transistor T1. The transistors T1, T2 and T3 areeach supported by a FD-SOI substrate (see, FIG. 1A). In thisimplementation, the back gates B1, B2 and B3 for the transistors T1, T2and T3, respectively, are directly electrically connected to the groundvoltage GND. Furthermore, the body (i.e., the channel region of thesemiconductor film layer) of the transistor T1, the body (i.e., thechannel region of the semiconductor film layer) of the transistor T2 andthe body (i.e., the channel region of the semiconductor film layer) ofthe transistor T3 are left floating. In addition, the transistors T1 andT3 are fabricated to have a thicker gate oxide. For example, the gateoxide of transistors T1 and T3 in the module 3 i have thicknesses ofabout 5 nm, while the gate oxide of transistor T1 in the module 3 c hasa thickness of about 1.5 nm. Increasing the thickness of the gate oxidesof transistors T1 and T3 has the effect of improving the gate-body (GB)and drain-body (DB) leakage and sub-threshold voltage performance.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIG. 20 illustrates operation of the module 3 i as a function of thetemperature, within a range between −150° C. and 200° C., with VDD=0.9V.The curve CB2″ shows results for implementation of the module 3 i. Forcomparison purposes, the curve CB2′ for the operation of module 3 f isalso shown. It will be noted that module 3 i exhibits substantially thesame linearity with a slight improvement (i.e., increase) in slope. Themodule 3 i is accordingly an improvement over the module 3 f.

In connection with fabrication of the circuit module for a partiallydepleted semiconductor on insulator (PD-SOI) substrate technology node,FIG. 21 shows a circuit diagram for an embodiment of a temperaturesensing circuit module 3 j. In this embodiment, the thermistor 4 isimplemented using the transistor T1 coupled in series with a transistorT3 (comprising, for example, an NMOS transistor) having a source S3 thatis coupled to the output terminal BS of the circuit module 3 and a drainD3 and a gate G3 that are electrically shorted to each other and coupledto the source S1 of transistor T1. The transistors T1, T2 and T3 areeach supported by a PD-SOI substrate (see, FIG. 1A). In thisimplementation, the back gates B1, B2 and B3 for the transistors T1, T2and T3, respectively, are directly electrically connected to the groundvoltage GND. Furthermore, the body (i.e., the channel region of thesemiconductor film layer) of the transistor T1, the body (i.e., thechannel region of the semiconductor film layer) of the transistor T2,and the body (i.e., the channel region of the semiconductor film layer)of the transistor T3 are also directly electrically connected to theground voltage GND. In addition, the transistors T1 and T3 arefabricated to have a thicker gate oxide. For example, the gate oxide oftransistors T1 and T3 in the module 3 j have thicknesses of about 5 nm,while the gate oxide of transistor T1 in the module 3 d has a thicknessof 1.8-2 nm. Increasing the thickness of the gate oxide of transistorsT1 and T3 has the effect of improving the gate-body (GB) and drain-body(DB) leakage and sub-threshold voltage performance.

In an alternative implementation shown by dotted lines, the drain D2 ofthe second transistor T2 may instead be coupled to a supply (bias)voltage, (for example, 1.0V) instead of to the ground voltage GND. Thisimplementation is made at the expense of an increase in the powerconsumption of the second transistor T2, and thus may be undesirable insome circuit implementations.

FIGS. 22A-22B illustrate operation of the module 3 j as a function ofthe temperature, within a range between −150° C. and 200° C., withVDD=1.2V. The curve CB5′″ shows results for implementation of the module3 j at the technology node H9 SOI. The curve CB6′″ shows results forimplementation of the module 3 j at the technology node 65 SOI. Forcomparison purposes, the curve CB2 for module 3 and the curves CB5″ andCB6″ for module 3 g are also shown. It will be noted that there is someimprovement in slope, however, the curve CB5′″ and CB6″ show an increasein the square coefficient of the second order polynomial trend line. Themodule 3 j is accordingly an improvement over the module 3 g and isuseful in some device applications.

To summarize in terms of best circuit module for each technology node:

a) for the technology node 40 bulk, the module 3 e provides the bestlinearity, while the module 3 h provides acceptable linearity withincreased slope (to support better precision);

b) for the technology node 65 bulk, the module 3 e provides the bestlinearity, while the module 3 h provides acceptable linearity withincreased slope (to support better precision);

c) for the technology node 65 SOI, the module 3 d provides the bestlinearity, while the module 3 j provides acceptable linearity withincreased slope (to support better precision);

d) for the technology node H9 SOI, the module 3 e provides the bestlinearity, but the module 3 h provides acceptable linearity withincreased slope (to support better precision); and

e) for the technology node 28 FD-SOI, the module 3 i provides the bestperformance in terms of both linearity and increased slope.

Reference is now made to FIG. 23 which shows a circuit diagram for anembodiment of a temperature sensing circuit module 3 k. The module 3 kis substantially the same as the module 3 c of FIG. 7 except that thebody (bulk) of the transistor T1 receives a bias voltage VB# generatedby a bias voltage source 2 (instead of being connected to ground). Thebias voltage source 2 is configured to generate the bias voltage VB#over a range of bias voltages; for example, from −1.1 V to +1.1 V forimplementation at the technology node 40 bulk, and from −1.2 V to +1.2 Vfor implementation at the technology node 65 bulk. With respect toimplementation of the module 3 k at the technology node 40 bulk, FIG. 24shows the effect that varying the bias voltage VB# has on the operationof the module 3 k as a function of the temperature. With respect toimplementation of the module 3 k at the technology node 65 bulk, FIG. 25shows the effect that varying the bias voltage VB# has on the operationof the module 3 k as a function of the temperature. It will be notedthat the application of certain voltage values for the bias voltage VB#can produce a more linear voltage versus current response of the module3 k and additionally shift the range of the output voltage.

Reference is now made to FIG. 26 which shows a circuit diagram for anembodiment of a temperature sensing circuit module 3 m. The module 3 mis substantially the same as the module 3 d of FIG. 9 except that thebody of the transistor T1 receives a bias voltage VB# generated by abias voltage source 2 (instead of being connected to ground). The biasvoltage source 2 is configured to generate the bias voltage VB# over arange of bias voltage from −1.2 V to +1.2 V. With respect toimplementation of the module 3 m at the technology node H9 SOI, FIG. 27shows the effect that varying the bias voltage VB# has on the operationof the module 3 m as a function of the temperature. With respect toimplementation of the module 3 m at the technology node 65 SOI, FIG. 28shows the effect that varying the bias voltage VB# has on the operationof the module 3 m as a function of the temperature. It will be notedthat the application of certain voltage values for the bias voltage VB#can produce a more linear voltage versus current response of the module3 m and additionally shift the range of the output voltage. It willfurther be noted that there is no change in performance for negativevalues of the bias voltage VB# in FIG. 28 (i.e., the performance fornegative values is substantially the same as the performance forVB#=0.0V).

Reference is now made to FIG. 29 which shows a circuit diagram for anembodiment of a temperature sensing circuit module 3 n. The module 3 nis substantially the same as the module 3 h of FIG. 17 except that thebody of each of the transistors T1 and T3 receives a bias voltage VB#generated by a bias voltage source 2 (instead of being connected toground). The bias voltage source 2 is configured to generate the biasvoltage VB# over a range of bias voltage; for example, from −1.1 V to+1.1 V for implementation at the technology node 40 bulk, and from −1.2V to +1.2 V for implementation at the technology node 65 bulk. Withrespect to implementation of the module 3 n at the technology node 40bulk, FIG. 30 shows the effect that varying the bias voltage VB# has onthe operation of the module 3 n as a function of the temperature. Withrespect to implementation of the module 3 n at the technology node 65bulk, FIG. 31 shows the effect that varying the bias voltage VB# has onthe operation of the module 3 n as a function of the temperature. Itwill be noted that the application of certain voltage values for thebias voltage VB# can produce a more linear voltage versus currentresponse of the module 3 n and additionally shift the range of theoutput voltage.

Reference is now made to FIG. 32 which shows a circuit diagram for anembodiment of a temperature sensing circuit module 3 o. The module 3 ois substantially the same as the module 3 j of FIG. 21 except that thebody of the transistors T1 and T3 receives a bias voltage VB# generatedby a bias voltage source 2 (instead of being connected to ground). Thebias voltage source 2 is configured to generate the bias voltage VB#over a range of bias voltage from −1.2 V to +1.2 V. With respect toimplementation of the module 3 o at the technology node H9 SOI, FIG. 33shows the effect that varying the bias voltage VB# has on the operationof the module 3 o as a function of the temperature. With respect toimplementation of the module 3 o at the technology node 65 SOI, FIG. 34shows the effect that varying the bias voltage VB# has on the operationof the module 3 o as a function of the temperature. It will be notedthat the application of certain voltage values for the bias voltage VB#can produce a more linear voltage versus current response of the module3 o and additionally shift the range of the output voltage. It willfurther be noted that there is no change in performance for negativevalues of the bias voltage VB# in FIG. 34 (i.e., the performance fornegative values is substantially the same as the performance forVB#=0.0V.

Reference 1 in FIG. 35 denotes an electronic appliance, in this case,for example, a cellular mobile telephone designed to withstand extremeenvironmental conditions, such as a significant temperature variation.

The electronic appliance 1 includes functional circuitry (for example,for implementing processing, communications, display, interface, etc.functions) wherein the functional circuitry includes a temperaturesensor in the form of one of the circuit modules 3 c-3 o as describedabove. In the context herein, the terms “substantially” or “about” willbe understood to mean within ±5% of an indicated value.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims.

1. A temperature sensing circuit, comprising: a thermistor comprising afirst MOS transistor having a first drain, a first source, a first gateand a first body, wherein the first MOS transistor is configured tooperate in weak inversion mode with a negative temperature coefficientdrain-source resistance; an output terminal coupled to the first sourceand configured to generate a positive temperature coefficient outputvoltage; and a current source comprising a second MOS transistor havinga second drain, a second source, a second gate and a second body,wherein the second gate is coupled to the first source and configured toimpose a drain-source current of the first MOS transistor; and whereinthe first body and second body are directly electrically connected to aground node.
 2. The temperature sensing circuit of claim 1, wherein thefirst and second MOS transistors are supported by a bulk substrate thatprovides the first and second bodies.
 3. The temperature sensing circuitof claim 1, wherein the first and second MOS transistors are supportedby a semiconductor on insulator (SOI) substrate, said SOI substrateincluding semiconductor film layer over a buried oxide layer over asemiconductor substrate, and wherein the semiconductor film layerprovides the first and second bodies.
 4. The temperature sensing circuitof claim 3, wherein the SOI substrate is one of a fully depleted SOIsubstrate or a partially depleted SOI substrate.
 5. The temperaturesensing circuit of claim 3, wherein the first MOS transistor further hasa first back gate and the second MOS transistor further has a secondback gate, and wherein the first and second back gates are providedwithin the semiconductor substrate of the SOI substrate.
 6. Thetemperature sensing circuit of claim 5, wherein the first and secondback gates are directly electrically connected to the ground node. 7.The temperature sensing circuit of claim 1, wherein the first drain isdirectly electrically connected to a supply voltage node.
 8. Thetemperature sensing circuit of claim 1, wherein the second source andsecond drain are directly electrically connected to the ground node. 9.The temperature sensing circuit of claim 1, wherein the second source isdirectly electrically connected to the ground node and the second drainis configured to receive a bias voltage.
 10. The temperature sensingcircuit of claim 1, wherein the first gate is directly electricallyconnected to the first drain.
 11. The temperature sensing circuit ofclaim 1, wherein the first gate of the first MOS transistor has a firstgate oxide thickness, and wherein the second gate of the second MOStransistor has a second gate oxide thickness, and wherein the first gateoxide thickness is thicker than the second gate oxide thickness.
 12. Thetemperature sensing circuit of claim 11, wherein the first and secondMOS transistors are supported by a bulk substrate providing the firstand second bodies.
 13. The temperature sensing circuit of claim 11,wherein the first and second MOS transistors are supported by asemiconductor on insulator (SOI) substrate, said SOI substrate includingsemiconductor film layer over a buried oxide layer over a semiconductorsubstrate, and wherein the semiconductor film layer provides the firstand second bodies.
 14. The temperature sensing circuit of claim 13,wherein the SOI substrate is a fully depleted SOI substrate or apartially depleted SOI substrate.
 15. The temperature sensing circuit ofclaim 13, wherein the first MOS transistor further has a first back gateand the second MOS transistor further has a second back gate, andwherein the first and second back gates are provided within thesemiconductor substrate of the SOI substrate.
 16. The temperaturesensing circuit of claim 15, wherein the first and second back gates aredirectly electrically connected to the ground node.
 17. The temperaturesensing circuit of claim 11, wherein the first drain is directlyelectrically connected to a supply voltage node.
 18. The temperaturesensing circuit of claim 11, wherein the second source and second drainare directly electrically connected to the ground node.
 19. Thetemperature sensing circuit of claim 11, wherein the second source isdirectly electrically connected to the ground node and the second drainis configured to receive a bias voltage.
 20. The temperature sensingcircuit of claim 11, wherein the first gate is directly electricallyconnected to the first drain.
 21. The temperature sensing circuit ofclaim 1, wherein the thermistor further comprises: a third MOStransistor coupled in series with the first MOS transistor and having athird drain which is coupled to the first source, a third source coupledto the output terminal, a third gate and a third body, wherein the thirdMOS transistor is configured to operate in weak inversion mode with anegative temperature coefficient drain-source resistance; and whereinthe third body is directly electrically connected to the ground node.22. The temperature sensing circuit of claim 21, wherein the first,second and third MOS transistors are supported by a bulk substrateproviding the first, second and third bodies.
 23. The temperaturesensing circuit of claim 21, wherein the first, second and third MOStransistors are supported by a semiconductor on insulator (SOI)substrate, said SOI substrate including semiconductor film layer over aburied oxide layer over a semiconductor substrate, and wherein thesemiconductor film layer provides the first, second and third bodies.24. The temperature sensing circuit of claim 23, wherein the SOIsubstrate is a fully depleted SOI substrate or a partially depleted SOIsubstrate.
 25. The temperature sensing circuit of claim 23, wherein thefirst MOS transistor further has a first back gate, the second MOStransistor further has a second back gate and the third MOS transistorfurther has a third back gate, and wherein the first, second and thirdback gates are provided within the semiconductor substrate of the SOIsubstrate.
 26. The temperature sensing circuit of claim 25, wherein thefirst, second and third back gates are directly electrically connectedto the ground node.
 27. The temperature sensing circuit of claim 21,wherein the first drain is directly electrically connected to a supplyvoltage node.
 28. The temperature sensing circuit of claim 21, whereinthe second source and second drain are directly electrically connectedto the ground node.
 29. The temperature sensing circuit of claim 21,wherein the second source is directly electrically connected to theground node and the second drain is configured to receive a biasvoltage.
 30. The temperature sensing circuit of claim 21, wherein thefirst gate is directly electrically connected to the first drain andwherein the third gate is directly electrically connected to the thirddrain.
 31. The temperature sensing circuit of claim 21, wherein thefirst gate of the first MOS transistor has a first gate oxide thickness,wherein the second gate of the second MOS transistor has a second gateoxide thickness, wherein the third gate of the third MOS transistor hasa third gate oxide thickness, and wherein the first and third gate oxidethicknesses are thicker than the second gate oxide thickness.